Contactless seals, in particular labyrinth seals, are widely used for sealing rotating systems in turbomachine construction. Because of the aerodynamic boundary layers which form, a high frictional power appears in the separating gap through which fluid flows between the rotating and stationary parts. This causes heating of the fluid in the separating gap and therefore also causes heating of the components surrounding the separating gap. The high material temperatures cause a reduction in the life of the corresponding components.
Depending on their design, exhaust gas turbochargers have an axial thrust from the exhaust gas turbine which acts against or in the same direction as that from the centrifugal compressor. In the latter case, the resulting pressure in the separating gap between the rotating rear wall of the compressor impeller and the adjacent stationary compressor casing has to be reduced. For this reason, such separating gaps have very tight tolerances. In addition, they usually have a contactless seal. Such narrow separating gaps involve a particularly high frictional power. In addition, the deflection and the eddying of the working fluid flowing through the separating gap lead to repeated mixing of the working fluid at the throttle locations of the seal and this is associated with a high level of momentum and heat exchange. Downstream of the throttle location, the working fluid has to be accelerated afresh each time in the peripheral direction on the rotating component so that the frictional power, and therefore the generation of heat, increases further in this region.
A cooling appliance for centrifugal compressors with sealing elements arranged on the rear wall of the compressor impeller, in the separating gap between the latter and the compressor casing, is known from EP 0 518 027 B1. In this arrangement, a cold gas which is provided with a pressure which is higher than that present at the outlet from the compressor impeller is fed through the seal. This gas impinges on the rear wall of the compressor impeller and simultaneously acts there as sealing air to prevent a flow of hot compressor air from the outlet of the compressor impeller through the labyrinth gap. The service life of such a compressor wheel provided with sealing geometry can be markedly increased by this means. In this solution, it is found to be a disadvantage that the specially shaped seal complicates the overall design and the assembly of the compressor and makes it more expensive. Because the clearance of the separating gap is in the range of tenths of a millimeter, furthermore, there is always a latent danger of the rotating compressor impeller rubbing on the compressor casing.
In contrast to this, no reduction in pressure in the separating gap is necessary in the case of an axial thrust of the exhaust gas turbine acting against the centrifugal compressor so that its clearance is in the range of millimeters and it becomes unnecessary to seal the separating gap in the region of the rear wall of the compressor impeller. A centrifugal compressor without such sealing elements is known from DE 195 48 852. It is simple in construction and therefore can be manufactured at favorable cost. There is no danger of the rotating compressor impeller rubbing against the compressor casing. Nevertheless, even in this case the frictional heat resulting from aerodynamic shear layers on the rear wall of the compressor impeller ensures heating of the compressor impeller and, therefore, a reduction in its life. No solution for reducing the generation of heat in the case of centrifugal compressors without sealing elements in the region of the rear wall of the compressor impeller is known.